This study is concerned with the transition of the boundary layer adjacent to a sidewall of a differentially heated cavity to a double-layer structure and with the important flow and heat transfer features of the double-layer structure. The transition to the double-layer structure is examined by comparing numerical and experimental results. The mechanism responsible for the formation of the double-layer structure, i.e. the stratification of the fluid in the core, is discussed. Detailed discussion of the energy balance adjacent to the sidewall reveals that the heat transfer in the double-layer structure is rather complex. There is an opposing horizontal thermal diffusion within the double-layer structure. (C) 2008 Elsevier Ltd. All rights reserved.